1. Technical Field of the Invention
The present invention relates in general to communication systems, and in particular, to wavelength division multiplexing (WDM) fiber-optic communication systems.
2. Description of Related Art
In fiber-optic communication systems, wavelength-division multiplexing is commonly used to multiplex multiple optical carrier signals (channels) onto a single optical fiber by using different wavelengths (colors) of laser light to carry different signals. This enables networks to not only add capacity, but also to provide bidirectional communication independent of traffic protocol or speed over one strand of fiber. In long haul WDM systems where the optical link is greater than 50-100 km, optical amplifiers are typically used to overcome fiber span losses. Optical amplifiers can be operated in either a constant-gain mode or a constant-power mode. In constant-power mode, the amplified output power is regulated to a fixed value, largely independent of the input signal power.
As the WDM channels propagate through the system, the gain and amplified spontaneous emission (ASE) noise applied to each of the constituent WDM channels varies slightly. While this variation is generally relatively small for signals passing through a single amplifier or fiber section, the effect becomes significant for a cascaded series of amplifiers typically found in WDM networks. Without some form of compensation, the cumulative effect produces an output spectrum with widely varying optical signal-to-noise ratio (OSNR) and power levels associated with each of the constituent WDM signals. Consequently, some WDM channels will reach the far end of the network with relatively poor operating margins, while others will arrive with significantly more margin than necessary.
Therefore, in many WDM systems, channel balancing is performed to adjust the relative input power levels (transmitter output power levels) of the constituent WDM channels until all of the associated outputs have nearly the same OSNR with optical power levels that satisfy the dynamic range requirements of the receivers. In other words, the input signal levels are adjusted to achieve relatively balanced OSNR levels across the WDM channels at the far end receivers. The process is also referred to as “pre-emphasis”, because the power levels are “pre-emphasized” at the transmitters to anticipate the variations in additive noise and gain across the spectrum as the signals pass through the optical amplifiers to the far end receivers.
One way of performing channel balancing is to use an optical spectrum analyzer (OSA) to directly measure the OSNR of each of the optical channels. However, OSA equipment is relatively expensive. In addition, the purpose of channel balancing is not necessarily to achieve balanced OSNR, but to provide optimal and consistent margin against bit errors across all of the constituent WDM channels. However, a channel balancing exercise based on Bit Error Ratio (BER) requires a significant amount of time that increases linearly with channel count since existing automatic pre-emphasis techniques measure the operating limits of each channel by modifying the operating point of one channel at a time to minimize the change in over all loading of the system during the balancing process. Therefore, what is needed is a cost-effective channel balancing mechanism with minimal time requirements.